1. Field of the Invention
This invention relates generally to a composite structure and, more specifically, to a broadband radome fabricated using inorganic polymers for use primarily in conjunction with missiles designed for flights of short duration during which the temperature of the radome wall is substantially elevated.
2. Brief Description of the Prior Art
Prior art broadband radomes for missiles, which have been designed primarily for one time flights of short duration, generally in the range of from about 0.5 to about 2 minutes, must have very thin skins (less than about 0.06 inches or 0.025 centimeters) in order to handle all frequencies below 18 GHz. which may be encountered. Such radomes have generally been fabricated by using polyimide resin reinforced with low dielectric (less than about 3) and low loss tangent value (less than about 0.02), generally fused silica (quartz) cloth with three to six layers of laminate for a skin or shell. The radome wall skin may comprise either a single 0.06 inch thick skin or plural skins, each skin having a thickness of up to 0.06 inches. The radome wall can also comprise two such skins spaced apart by a polyimide glass honeycomb. Wire grids for electromagnetic tuning may also be placed inside or on the surface of one or more skins of the radome. Radomes of this type have been used for the radome of the HARM missile and for other prototype missile radomes. The weakness of this design is that it is designed for applications wherein the wall temperatures generated are limited to up to about 1200.degree. F. to 1400.degree. F. for the contemplated short flights of about 0.5 to about 2 minutes. Such prior art missiles have also been subjected to speeds of up to about Mach 4 with the radome walls being designed to handle temperatures only up to those attained during flights of the above described types.
New generation missiles of the above described type have more stringent requirements, among these being the ability to operate for longer periods of time, the ability to operate at higher temperatures or a combination thereof. The next generation of missiles is now being designed to operate up to speeds of about Mach 6 for periods of up to about 5 minutes with the radome walls expected to encounter temperatures of about 1600.degree. F. for an extended period of the flight with peak temperatures up to about 2300.degree. F. for several seconds. The organic polymers used in the prior art have a problem under these conditions in that the polymer pyrolyzes and causes carbon to form at the higher temperatures to be encountered by the radomes now being designed, thereby defeating the transmissive properties required by the radomes. It is therefore apparent that improved radome materials are required for the radomes now being designed which are capable of operation over the entire flight and under the extreme temperature conditions to be encountered.